Low pressure sodium (LPS) is the astronomers'
first choice of light source.

Full cut off fixtures are essential whatever
light source is used.

The UK already has a large installed base
of LPS lamps but in wasteful fittings.

Much light (up to 30%) is presently wasted
directly into the skies.

Simple improvements would save energy
and help astronomers.

Dreadful external lighting fixtures are
still in common use in the UK.

A code is needed to educate installers
and users how to use lighting properly.

I believe that the problem of badly designed
street lighting and security lighting could be quantified and
measured in such a way as to be legally enforceable.

I am a long-time amateur astronomer and former
radio astronomer. I have an interest in control of light pollution
from the perspective of amateur astronomers and in filters and
luminaire designs that can help to reduce the problem. I should
declare an interest here, my own Nonad photographic filter is
one such doped glass filter, Baader planetarium also sell a Neodymium
filter effective against LPS. Orion Optics (UK) make a visual
Sodium light interference filter effective against mixed HPS/LPS
lighting.

The questions that I feel able to address are
as follows. I will include some background material (not printed)
as well as web references to material in US light pollution statutes
that may provide a useful model.

Impact of light pollution on UK astronomy?

It is now difficult to get away from light pollution
anywhere in the UK. The level of street lighting and the increasing
move to illuminate all road junctions on major roads makes it
very difficult to find truly dark skies. Long photographic exposures
even in good conditions suffer from scattered light from nearby
cities up to 20 miles away (or more). Professional optical astronomy
is now limited to doing experiments that are possible under light
polluted conditions and testing equipment for use overseas.

Light pollution is itself perhaps something
of a misnomer. "Wasted light" would be a less emotive
descriptionlight that goes directly into the sky would
be valuable if it were reflected downwards.

Is it measurable in such a way as to be legally
enforceable?

It should be possible to measure and in many
cases determine by simple visual inspection whether or not a particular
design of luminaire will be sky friendly or not. The vast majority
of the UK street lamps are by design sending a significant proportion
of their light skywards. This is inefficient.

It could even be partly resolved by retro fitting
relatively simple designs of reflector into old lamps. The simplest
designs of reflector need be no more complex or heavy than aluminium
pie trays.

There are some clearly measurable properties
of luminaires.

1. Power consumption.

2. Total light output.

3. Proportion that is wasted.

4. Spectrum of the light.

The aims for astronomy are:

1. Use the minimum amount of light needed
for the task in hand.

2. Minimise the amount of light that goes
upwards above the horizontal.

3. Use only light which can be relatively
easily filtered out at the telescope.

A lot of commercial properties are significantly
over lit because it is believed (perhaps correctly) that the glare
of bright lights attracts passing trade. Many people find it hard
to distinguish between good lighting that provides good night
visibility and bad lighting that dazzles with glare.

Several US regions have recommendations and
in some cases enforcement of good lighting codes. One such example
is Bradford, Connecticut (ref 2). It is fairly obvious why full
cut-off is preferable.

A problem in the UK is that there are fewer
full cut-off designs available compared to other European countries.
This can be easily confirmed by looking down from an aircraft
in the UK. The dazzling bright lights seen along the streets are
caused by direct line of sight to the lamp. In a region with full
cut-off lighting you will instead see only the diffuse pools of
light under the streetlamp.

The third option is to choose a type of light
that astronomers can easily filter out. This is possible provided
that nearly monochromatic light source is used. The most common
one being the familiar orange-yellow low pressure sodium lamps
(about the most energy efficient light source).

Areas near to observatories should use a combination
of lower ambient light levels (perhaps even time- switched to
off for some period like 1am-3am), and shielded optical design
full cut-off luminaries fitted with low pressure sodium lights.
This is about as good as you can hope for.

In the vicinity of optical observatories additional
measures to ban poorly designed and installed lighting may also
be needed. San Diego, Tucson and a few other US cities near to
major optical observatories will provide a model for what may
be achieved and what sort of opposition may be encountered. The
US still uses mostly mercury street lighting and opposition to
low pressure sodium lights is a lot more vociferous because they
are associated mainly with junkyards.

US recommendations and solutions may not always
be right for the UK thoughbecause the bulk of the US street
lighting installed base are old mercury lamps (about half the
energy efficiency). La Palma is perhaps a model more relevant
to UK experience with a large installed base of LPS and professional
observatoriesChris Benn has published some work on spectroscopy
at the ING and the influence of light pollution on the island.
Photographs there show the improvement obtained by switching to
shielded street lighting with before and after shots of some street
scenes (Ref 3).

The minimum aim should be that all new luminaries
used for routine street lighting of whatever type should conform
to certain minimum standards for stray light. I would suggest
full cut-off designs with a tolerance of about 10 degrees extra
margin to allow for mechanical installation misalignment.

This treatment is simplistic but serves to illustrate
an astronomer's point of view.

Targets for a well-designed full cut-off luminaire:

No light goes upwards

Lamp is fully shielded

Simple plain glass cover

Tolerant of small installation errors

Uniformity of illumination along the road

Shaped reflectors

Maximise ratio of pole separation to height.

Minimise power consumption

Physical size minimised

Weight

Wind loading

Principal types of lamp and characteristics
of a typical 100W lamp (ref 1).

Name

Type

Efficiency
lumen/W

Lifetime
MTBF/hrs

Colour Temp/K

Colour

Incandescent

Continuum

13

1000

2700

White

Tungsten halogen

Continuum

20

3000

3200

White

Mercury

Line +
Phosphor

50

20000

3000*

Blue White

Metal Halide/
HID

Continuum

80

10000

3500

White

HPS (SON)

Broadband

95

30000

2000*

Peachy white

LPS (SOX)

Narrow line

200

20000

N/a

Orange-
Yellow

* Colour temperature is imprecise, the emission characteristics
are not truly like a hot black body.

In the UK it is likely that there will be very few mercury
lamps and the vast majority of street illumination will be from
HPS and LPS with a few HID lamps in prime regions of city centres.

The HPS lamp has become the favourite for most new lighting
schemes in the UK. Some locations do use full cut-off luminaries,
but by no means all of them. This is regrettable as HPS light
is fairly broadband and so cannot be adequately filtered out at
the telescope.

Here is a rough model of the contributions to light pollution
for HPS and LPS with and without filters:

HPS

HPS full cut-off

LPS

LPS full cut-off

Downwards

80

97

70

97

Reflected up

8

10

7

10

Upwards

20

3

30

3

Net skywards

28

13

37

13

Filter factor

1/10

1/10

1/200

1/200

Net after filter

3

1.3

0.2

<0.1

These numbers are of necessity approximate and the light
reflected upwards depends on the reflectivity of the road surface
(typically about 8%), grass (10%) and pavement (typically 15%).
Light concrete roads will be worse and very dark bitumen rich
roads will be better. Snow is disastrous but rare.

The numbers may be modified in compact urban areas because
light escaping directly from the top of a lamppost is virtually
unimpeded, whereas some of the light reflected from the street
level is shaded from the sky by buildings and is therefore of
decreased intensity.

Filters are assumed to be the optimal ones available to amateurs
(Orion UK for HPS and Nonad or Baader for LPS). Professional optical
observatories may be able to do slightly better.

Few UK amateur astronomers are aware how easily the LPS light
can be filtered. (Plates 1 and 4) (not printed).

The standard 1960's LPS street lamp is not much better with
ribbed glass and a crude fixture that allows around 30% of the
light to go directly skywards. The lamps are so efficient that
this wasn't considered to be a problem when they were installed
(Figure 1). Belgian LPS fixtures are by comparison full cut-off
designs with internal reflectors to control the light (Plates
2 and 3). (not printed)

There are some very bad luminaire designs where more than
half the light goes skywards. Typically used in supermarket car
parks and appear to be designed to generate sky glow to attract
passing trade. A classic bad design looks like a goldfish bowl
with a lamp in on top of a vertical stalk (Figure 2). (not printed)

The other extremely bad design is the symmetric parabolic
security lamp (500W quartz halogen). These have disastrous light
spill and glare problems and there is no excuse at all for thisasymmetric
designs give better control, a more useful light distribution
and vastly reduced glare (Figure 3). (not printed)

CONCLUSION

To make astronomers happy:

Use the right amount of light

Light only the right placesshielded full cut-off
fixtures are best

Use the right type of lightlow pressure sodium
is easily filtered

Ideally we would like to see all new fixtures to match all
these criteria. Wasted light is wasted money.

It would be nice to see some of the old inefficient luminaires
retrofitted with inexpensive aluminium foil reflectors so that
more of their light goes down onto the road instead of skywards.